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          Linux权能与PAM机制
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        <blockquote>
<p>实验环境：<code>uname -a</code></p>
<p>​      <code>Linux ubuntu 4.4.0-121-generic #145-Ubuntu SMP Fri Apr 13 13:47:23 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux</code></p>
</blockquote>
<hr>
<h3 id="权能对应的系统调用"><a href="#权能对应的系统调用" class="headerlink" title="权能对应的系统调用"></a>权能对应的系统调用</h3><p>使用 <code>sudo find / -name capability.h</code> 查找到路径 <code>/usr/src/linux-headers-4.4.0-121/include/uapi/linux/capbility.h</code>  ，整理可得下表：</p>
<a id="more"></a>

<table>
<thead>
<tr>
<th>权能</th>
<th>编号(相关系统调用）</th>
<th>解释</th>
</tr>
</thead>
<tbody><tr>
<td>CAP_CHOWN</td>
<td>0(chown)</td>
<td>对文件的UIDs和GIDs做任意修改</td>
</tr>
<tr>
<td>CAP_DAC_OVERRIDE</td>
<td>1</td>
<td>忽略对文件的DAC访问限制</td>
</tr>
<tr>
<td>CAP_DAC_READ_SEARCH</td>
<td>2</td>
<td>忽略DAC中对文件和目录的读、搜索权限</td>
</tr>
<tr>
<td>CAP_FOWNER</td>
<td>3</td>
<td>忽略进程UID与文件UID的匹配检查</td>
</tr>
<tr>
<td>CAP_FSETID</td>
<td>4</td>
<td>文件修改时不清除setuid和setgid位，不匹配时设置setgid位</td>
</tr>
<tr>
<td>CAP_KILL</td>
<td>5(kill)</td>
<td>绕过发送信号时的权限检查</td>
</tr>
<tr>
<td>CAP_SETGID</td>
<td>6(setgid)</td>
<td>设置或管理进程GID</td>
</tr>
<tr>
<td>CAP_SETUID</td>
<td>7(setuid)</td>
<td>管理或设置进程UID</td>
</tr>
<tr>
<td>CAP_SETPCAP</td>
<td>8(capset)</td>
<td>允许授予或删除其他进程的任何权能</td>
</tr>
<tr>
<td>CAP_LINUX_IMMUTABLE</td>
<td>9(chattr)</td>
<td>允许设置文件的不可修改位(IMMUTABLE)和只添加(APPND-ONLY)属性</td>
</tr>
<tr>
<td>CAP_NET_BIND_SERVICE</td>
<td>10</td>
<td>允许绑定到小于1024的端口</td>
</tr>
<tr>
<td>CAP_NET_BROADCAST</td>
<td>11</td>
<td>允许socket发送监听组播</td>
</tr>
<tr>
<td>CAP_NET_ADMIN</td>
<td>12</td>
<td>允许执行网络管理任务</td>
</tr>
<tr>
<td>CAP_NET_RAW</td>
<td>13(socket)</td>
<td>允许使用原始套接字</td>
</tr>
<tr>
<td>CAP_IPC_LOCK</td>
<td>14(mlock)</td>
<td>允许锁定共享内存片段</td>
</tr>
<tr>
<td>CAP_IPC_OWNER</td>
<td>15</td>
<td>忽略IPC所有权检查</td>
</tr>
<tr>
<td>CAP_SYS_MOUDLE</td>
<td>16(init_module)</td>
<td>插入和删除内核模块</td>
</tr>
<tr>
<td>CAP_SYS_RAWIO</td>
<td>17</td>
<td>允许对ioperm/iopl的访问</td>
</tr>
<tr>
<td>CAP_SYS_CHROOT</td>
<td>18(chroot)</td>
<td>允许使用chroot()系统调用</td>
</tr>
<tr>
<td>CAP_SYS_PTRACE</td>
<td>19(ptrace)</td>
<td>允许跟踪任何进程</td>
</tr>
<tr>
<td>CAP_SYS_PACCT</td>
<td>20(acct)</td>
<td>允许配置进程记账</td>
</tr>
<tr>
<td>CAP_SYS_ADMIN</td>
<td>21</td>
<td>允许执行系统管理任务</td>
</tr>
<tr>
<td>CAP_SYS_BOOT</td>
<td>22(reboot)</td>
<td>允许重新启动系统</td>
</tr>
<tr>
<td>CAP_SYS_NICE</td>
<td>23(nice)</td>
<td>允许提升优先级，设置其他进程优先级</td>
</tr>
<tr>
<td>CAP_SYS_RESOURCE</td>
<td>24(setrlimit)</td>
<td>设置资源限制</td>
</tr>
<tr>
<td>CAP_SYS_TIME</td>
<td>25(stime)</td>
<td>允许改变系统时钟</td>
</tr>
<tr>
<td>CAP_SYS_TTY_CONFIG</td>
<td>26(vhangup)</td>
<td>允许配置TTY设备</td>
</tr>
<tr>
<td>CAP_MKNOD</td>
<td>27(mknod)</td>
<td>允许使用mknod()系统调用，创建特殊文件</td>
</tr>
<tr>
<td>CAP_LEASE</td>
<td>28(fcntl)</td>
<td>为任意文件建立租约</td>
</tr>
<tr>
<td>CAP_AUDIT_WRITE</td>
<td>29</td>
<td>允许向内核审计日志写记录</td>
</tr>
<tr>
<td>CAP_AUDIT_CONTROL</td>
<td>30</td>
<td>启用或禁用内核审计，修改审计过滤器规则</td>
</tr>
<tr>
<td>CAP_SETFCAP</td>
<td>31</td>
<td>设置文件权能</td>
</tr>
<tr>
<td>CAP_MAC_OVERRIDE</td>
<td>32</td>
<td>允许MAC配置或状态改变，为smack LSM实现</td>
</tr>
<tr>
<td>CAP_MAC_ADMIN</td>
<td>33</td>
<td>覆盖强制访问控制</td>
</tr>
<tr>
<td>CAP_SYSLOG</td>
<td>34(syslog)</td>
<td>执行特权syslog(2)操作</td>
</tr>
<tr>
<td>CAP_WAKE_ALARM</td>
<td>35</td>
<td>触发将唤醒系统的东西</td>
</tr>
<tr>
<td>CAP_BLOCK_SUSPEND</td>
<td>36(epoll)</td>
<td>可以阻塞系统挂起的特性</td>
</tr>
<tr>
<td>CAP_AUDIT_READ</td>
<td>37</td>
<td>允许通过一个多播socket读取审计日志</td>
</tr>
</tbody></table>
<p>详情见<a href="http://man7.org/linux/man-pages/man7/capabilities.7.html" target="_blank" rel="noopener">man capabilities</a> 。</p>
<hr>
<h3 id="基于PAM用户权限设置系统"><a href="#基于PAM用户权限设置系统" class="headerlink" title="基于PAM用户权限设置系统"></a>基于PAM用户权限设置系统</h3><p>PAM 的全称为“可插拔认证模块（Pluggable Authentication Modules）”。设计的初衷是将不同的底层认证机制集中到一个高层次的API中，从而省去开发人员自己去设计和实现各种繁杂的认证机制的麻烦。</p>
<p>PAM认证一般遵循这样的顺序：Service(服务)→PAM(配置文件)→pam_*.so，PAM配置文件在<code>/etc/pam.d/</code> 目录下，原理图如下：</p>
<p><img src="pam.png" alt="pam"></p>
<p>PAM中配置字段：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">moudle_type  control_flag  moudle_path moudle_option&#x2F;moudle_args</span><br></pre></td></tr></table></figure>

<ol>
<li><code>module_type</code> 为 相应服务指定模块类型</li>
</ol>
<table>
<thead>
<tr>
<th>模块类型</th>
<th>说明</th>
</tr>
</thead>
<tbody><tr>
<td>auth(验证模块)</td>
<td>用于验证用户或设置/销毁凭证</td>
</tr>
<tr>
<td>account(帐户管理模块)</td>
<td>将执行与访问、帐户及凭证有效期、密码限制/规则等有关的操作</td>
</tr>
<tr>
<td>session(会话管理模块)</td>
<td>定义用户登录前的,及用户退出后所要进行的操作.如:登录连接信息,用户数据的打开与关闭,挂载文件系统等</td>
</tr>
<tr>
<td>passwd(密码管理模块)</td>
<td>将执行与密码更改/更新有关的操作</td>
</tr>
</tbody></table>
<ol start="2">
<li>control_flag 将指定模块的堆栈行为，配置文件中列出模块被调用的顺序</li>
</ol>
<table>
<thead>
<tr>
<th>控制标记</th>
<th>说明</th>
</tr>
</thead>
<tbody><tr>
<td>required</td>
<td>堆栈中的所有 required 模块必须看作一个成功的结果。如果一个或多个 required 模块失败，则实现堆栈中的所有 required 模块，但是将返回第一个错误</td>
</tr>
<tr>
<td>requisite</td>
<td>如果本模块没有被满足，那本次认证一定失败，而且整个栈立即中止并返回错误信号</td>
</tr>
<tr>
<td>sufficient</td>
<td>如果标记为 sufficient 的模块成功并且先前没有 required 或 sufficient 模块失败，则忽略堆栈中的所有其余模块并返回成功</td>
</tr>
<tr>
<td>optional</td>
<td>如果堆栈中没有一个模块是 required 并且没有任何一个 sufficient 模块成功，则服务/应用程序至少要有一个 optional 模块成功</td>
</tr>
<tr>
<td>include</td>
<td>包含其他规则（服务），文件嵌套，可以互相调用，如：login，auth，system-auth等</td>
</tr>
<tr>
<td>[value1=action1  value2=action2 ….]</td>
<td>六种动作：ok ，done，bad，die，ignore，reset。</td>
</tr>
</tbody></table>
<p>​        Example：auth [user_unknown=ignore success=ok ignore=ignore default=bad] pam_securetty.so</p>
<ol start="3">
<li><code>module_path</code> 将指定实现模块的库对象的路径名称。默认情况下，它将被设为 <code>/lib/security</code>。        <img src="libpam.png" alt="libpam"></li>
<li>module_options/module_args（可选字段）将指定可以传递给服务模块的选项或实参。</li>
</ol>
<hr>
<p>实验：配置用户<code>userping</code> ，设置<code>cap_net_raw</code> 权能。</p>
<ul>
<li>添加用户<code>userping</code>  :  <code>sudo adduser userping</code> </li>
<li>查看并清除<code>/bin/ping</code> 的权能：</li>
</ul>
<p><img src="%E9%80%89%E5%8C%BA_057.png" alt="选区_057"></p>
<ul>
<li>切换到用户<code>userping</code> 再<code>ping</code> 无法<code>ping</code>   : <code>su userping</code> </li>
<li><code>userping</code> 登录时给<code>/bin/ping</code> 添加权能：</li>
</ul>
<figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#</span><span class="bash"><span class="comment">####################################################</span></span></span><br><span class="line"><span class="meta">#</span><span class="bash"> File Name: ping_cap.sh</span></span><br><span class="line"><span class="meta">#</span><span class="bash"> Author: xsyin</span></span><br><span class="line"><span class="meta">#</span><span class="bash"> mail: shouyinXu@163.com</span></span><br><span class="line"><span class="meta">#</span><span class="bash"> Created Time: 2018年04月30日 星期一 16时05分57秒</span></span><br><span class="line"><span class="meta">#</span><span class="bash"><span class="comment">###################################################</span></span></span><br><span class="line"></span><br><span class="line"><span class="meta">#</span><span class="bash">!/bin/sh</span></span><br><span class="line">[ "$PAM_TYPE" == "open_session" ] || exit 0</span><br><span class="line"></span><br><span class="line">if [ "$PAM_USER" == "userping" ]; then</span><br><span class="line">	setcap cap_net_raw=eip /bin/ping</span><br><span class="line">	echo "SUCCESS"</span><br><span class="line">else</span><br><span class="line">	echo "FAILURE"</span><br><span class="line">fi</span><br></pre></td></tr></table></figure>

<ul>
<li>将<code>ping_cap.sh</code> 移动到 <code>/usr/local/bin</code> 路径下，并设置为可执行：</li>
</ul>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">sudo chmod u+x &#x2F;usr&#x2F;loca&#x2F;bin&#x2F;ping_cap.sh</span><br></pre></td></tr></table></figure>

<ul>
<li>所有登录都使用<code>common-session</code> 模块：</li>
</ul>
<p><img src="%E9%80%89%E5%8C%BA_058.png" alt="选区_058"></p>
<p>因此在<code>common-session</code> 模块中添加规则: </p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">session  optional  pam_exec.so debug log&#x3D;&#x2F;tmp&#x2F;pam_exec.log seteuid &#x2F;usr&#x2F;local&#x2F;bin&#x2F;ping_cap.sh</span><br></pre></td></tr></table></figure>

<p>必须模块，开启了<code>debug</code>模式，用户<code>userping</code> 每次登录会执行<code>ping_cap.sh</code></p>
<ul>
<li>切换到<code>userping</code> 用户：</li>
</ul>
<p><img src="%E9%80%89%E5%8C%BA_059.png" alt="选区_059"></p>
<p>出错多次，最终调试成功：</p>
<p><img src="%E9%80%89%E5%8C%BA_061.png" alt="选区_060"></p>
<h3 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h3><p>优点：<code>userping</code> 用户无法执行特权命令，通过<code>pam</code> 可赋予一些特权操作。</p>
<p>缺点：本实验中修改了<code>ping</code> 的文件权能，影响了其他用户。退出<code>userping</code> 后，<code>ping</code> 命令变成有效了。</p>
<p>过渡C版本：</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/*************************************************************************</span></span><br><span class="line"><span class="comment">    &gt; File Name: ping_cap.c</span></span><br><span class="line"><span class="comment">    &gt; Author: xsyin</span></span><br><span class="line"><span class="comment">    &gt; Mail: shouyinXu@163.com </span></span><br><span class="line"><span class="comment">    &gt; Created Time: 2018年04月30日 星期一 14时56分35秒</span></span><br><span class="line"><span class="comment">    &gt; Make: gcc ping_cap.c -lcap -o ping_cap</span></span><br><span class="line"><span class="comment"> ************************************************************************/</span></span><br><span class="line"></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;stdio.h&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;stdlib.h&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;string.h&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;sys/types.h&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;unistd.h&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;sys/capability.h&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;sys/prctl.h&gt;</span></span></span><br><span class="line"></span><br><span class="line"><span class="meta">#<span class="meta-keyword">undef</span> _POSIX_SOURCE</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">extern</span> <span class="keyword">int</span> errno;</span><br><span class="line"></span><br><span class="line"><span class="keyword">const</span> <span class="keyword">char</span> *path_p = <span class="string">"/bin/ping"</span>;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">void</span> <span class="title">listCaps</span><span class="params">()</span></span>&#123;</span><br><span class="line">    <span class="keyword">cap_t</span> caps = cap_get_file(path_p);</span><br><span class="line">    <span class="keyword">ssize_t</span> y = <span class="number">0</span>;</span><br><span class="line">    <span class="built_in">printf</span>(<span class="string">"The file %s was give capabilities %s\n"</span>,path_p, cap_to_text(caps, &amp;y));</span><br><span class="line">    fflush(<span class="number">0</span>);</span><br><span class="line">    cap_free(caps);</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">int</span> <span class="title">main</span><span class="params">(<span class="keyword">int</span> argc, <span class="keyword">char</span> <span class="keyword">const</span> *argv[])</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">	<span class="keyword">uid_t</span> uid = getuid();</span><br><span class="line"></span><br><span class="line">	<span class="keyword">if</span>(uid != <span class="number">1000</span>) </span><br><span class="line">		<span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line"></span><br><span class="line">	<span class="built_in">printf</span>(<span class="string">"uid:%d\n"</span>, uid);</span><br><span class="line">	</span><br><span class="line">	<span class="keyword">cap_value_t</span> cap_values[] = &#123;CAP_NET_RAW&#125;;</span><br><span class="line">	<span class="keyword">cap_t</span> caps = cap_init();</span><br><span class="line">	cap_set_flag(caps,CAP_PERMITTED,<span class="number">1</span>,cap_values,CAP_SET);</span><br><span class="line">	cap_set_flag(caps,CAP_EFFECTIVE,<span class="number">1</span>,cap_values,CAP_SET);</span><br><span class="line">	cap_set_flag(caps,CAP_INHERITABLE,<span class="number">1</span>,cap_values,CAP_SET);</span><br><span class="line">	<span class="keyword">if</span>(cap_set_file(path_p,caps))</span><br><span class="line">		perror(<span class="string">"cap_set_file() ERROR: "</span>);</span><br><span class="line">	<span class="keyword">else</span></span><br><span class="line">		<span class="built_in">printf</span>(<span class="string">"success\n"</span>);</span><br><span class="line">	cap_free(caps);</span><br><span class="line">	listCaps();</span><br><span class="line"></span><br><span class="line">	<span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>缺陷：未搞清楚如何获取PAM正在授权的用户，无法判断切换至<code>userping</code> 用户时设置权能。</p>

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